Electromechanical transducer



Feb. 28, E950 c. R. HOLDEN 2,498,737

ELECTROMECHANICAL TRANsDUcER Filed June '7'. 1946 Inventor.- CHARLES REVEL L HOL DEN, DECEAS ED, BY CORA E. HOLDENJ EXECUTRIX.

Attorneys k Patented Feb. 28, 1950 Charles Revell Holden, deceased, late of Altadena, Calif., by Cora E. Holden, executrix, Altadena, Calif., assignor to William H. T. Holden, New

York, N. Y.

Application .lune 7, 1946, Serial No. 675,272

36 Claims. i y This invention relates to electromechanical transducers of the piezo-electric type for the conversion of electrical energy into acoustical energy in either gaseous or liquid fluids. The invention is particularly useful at high energy levels and at high frequencies where there is danger of shattering the piezo-electric crystals. Transducers of this type are used` in the treatment of gases or liquids for such purposes as emulsication, sterilization and other elects produced by supersonic vibrations at suitable energy levels.

An object of the invention is to provide an improved electromechanical transducer adapted for the conversion of electrical energy into mechanical energy, said electrical energy being supplied at high frequencies in the supersonic or higher audible ranges or in the radio frequency spectrum, as the case may be, and at relatively high energy levels.

A further object of the invention is to provide an improved construction for transducers of this type, permitting the use of piezo-electric crystals as the means of converting electrical energy into mechanical energy with increased emciency and with reduced tendency to cause shattering of the crystals.

A further object of the invention is to provide an improved and efficient closure for separating the chamber in which the electromechanical transformation occurs from the chamber in which a liquid or other substance is subjected to mechanical treatment by the high frequency energy.

Another object of the invention is to provide a member for separating the portion of the device utilized for treating the fluid from the portion in which the mechanical vibrations are produced by a member which in itself produces vibrations contributing to the total mechanical energy.

A further bject of the invention is to provide an improved-means for terminating a stack of piezo-electric crystals in which the amount of mechanical energy produced at the contact boundary between the termination and any other object will be minimized.

A further object of the invention is to provide means for adjusting the length of path of the acoustic waves produced in the uid medium to secure the most effective distribution of energy.

Still another object of the invention is the provision of a transducer capable of converting electrical energy into mechanical energy, or vice versa, in which a face of the transducing member is in engagement with a face of a reilecting member, the reflecting member having mechani- 2 cal or acoustic properties which cause the face engaging the transducing member to absorb relatively little mechanlcal energy.

Other and further objects will become apparent upon reading the specification.

The particular embodiment of the invention illustrated in the drawing shows a form of construction suitable for the treatment of fluids Whose electrical conductivity is suiciently great to permit the uid to act as an electrode transmittin'g energy to one end of the transducer. Where the fluid under treatment is suiciently non-conducting to prevent its use as an electrode, electrical contact with the crystal may be established by aluminizing a face of thecrystal orby any other suitable means.

Reference is now made to the drawings, in which:

Fig. 1 represents a sectional elevation of a transducer embodying the invention,

Fig. 2 shows an exploded perspective view of the crystal stack,

Fig. 3 shows a modification-.of Fig. l providing for adjustment of the path length of the acoustic waves, and

Fig. 4 represents a modified form of construction for plate t of Fig. 2.

Referring to Fig. l, the main housing of the transducer consists .of an upper section. I secured to a lower section 2 by means of bolts 3. An electrically conducting fluid to be treated enters by inlet t and leaves by outlet 5. A front crystal 6 is securedbetween gaskets 'l and-8, being clamped between the upper section l and lower section 2 of the housing by the action of bolts 3. These gaskets prevent leakage from either section of the housing.

Lower section 2 is filled with a circulating insulating oil which also serves to cool the stack of vibrating piezo-electric crystals. This oil enters section 2 at inlet 9 and leave's at outlet l0.

The crystal stack consists of front plate 8, intermediate crystals il, i2, and i3, and a rear reflector crystal I5, the crystals being assembled with additive polarity so that they all expand in response to one polarity and all contract in response to the opposite polarity.

Reector crystal l5 is shown as being one-half the thickness of vibratory crystals li, i2, and i3, but it may be any odd number of l/A, wavelengths, the vibratory crystals being an odd number of 1A wavelengths in thickness. The effective thickness of front plate 6 is the same as that of a vibratory crystal, that is eiectively an odd number of 1/2 wavelengths.

encarar "The housing is maintained at ground potential by conductor I8. An adjustable source I1 of high power at high frequency is provided, one terminal of source I'I being grounded through conductor IB and the high voltage terminal being connected by conductor I9 to a terminal screw 20. An electrode 2| is held in yielding engagement with reflector crystal I5 by the action of a compression spring 22. Electrode 2| also makes sliding electrical contact with terminal screw 20. Terminal screw 20 and electrode {ZI are insulated from housing section 2 by insulators 23 and 24 which engage gaskets 25 and 26 and are secured by the action of a nut 21 on terminal screw 20.

Between the reiector crystals I5 and the front crystal 6 is a plurality of individual vibratory crystals II, I2 and I3. 'I'hese vibratory crystals are arranged in a stack with a film of oil between adjacent crystals. Embedded in the electrode 2| as indicated at 29 is a plurality of vertically extending retaining members 30 formed of suitable insulating material.A

The reflector plate I5 is entirely coated with a thin electrical conducting layer. This may be accomplished by any suitable process, such as aluminizing. In this manner the electrical connection established by electrode IZ'I with the rear surface of reilector plate I5 is carried to the front surface of reflector plate I5 where it establishes contact with the rear surface of vibratory crystal I3. This also reduces any tendency on the part of the reflector crystal to generate second harmonics of the frequency of the vibratory crystals.

Thus, one terminal of sourcel II is connected to the rear of the vibratory crystal stack through conductor I9, terminal screw 2U, rear electrode 2l and the conducting surface of reilector crystal I5.

The other terminal is connected to the front surface of the stack through conductor I8 to ground and thence through conductor I6 to the metal housing section I of the unit. The front surface of the front crystal 6 will be in contact with the conducting solution under treatment, and its rear surface will be in contact with the front surface of vibratory crystal II. This will cause vibration of all the crystals of the stack including front plate 6 at the frequency generated by source Il which is adjusted to produce conditions approximating or the equivalent of resonance. All of the crystals, including front plate t and rear reiiector crystal I5 are X-cut, that is, cut to produce a mechanical deection in a direction parallel to the direction of the electric eld. In the case of reflector crystal I5, it is shielded by its conducting coating and hence, generates no mechanical vibrations.

As shown in Fig. 2, front plate 6 is provided with two annular grooves, one on each side, thereby providing a circular portion of reduced section, concentricV with the stack of vibratory crystals II, I2 and I3, as indicated at 28. An alternative arrangement is illustrated in Fig. 4, in which the entire central section of 6 is of reduced thickness.

In operation, the frequency of source I1 is adjusted to a value at or near the natural period of vibration of crystals II, I2, and I3. The dimensions of front plate 6 are so proportioned with respect to the dimensions of intermediate crystals II, I2, and I3 that its natural period of vibration will be as nearly as possible the same as that of the intermediate crystals, or an odd number of half wavelengths.

It will thus be seen that front crystal 6 not only divides the complete transducer unit into a treatment compartment and a transducing compartment but it'also brings a vibration generating surface directly into contact with the fluid being treated. This arrangement increases the efliciency of the unit as compared with transducers of this type in which the vibratory energy is applied to the iuid under treatment through a diaphragm. Such a diaphragm, when used to divide the unit into two compartments, absorbs vibratory energy which would otherwise be transmitted to the iluid under treatment.

In Fig. 3, provision is made for adjusting the path of travel of the acoustic waves generated in the fluid under treatment. A threaded member 3| is adjustably inserted in upper housing portionl and is provided with a nut 34 which may be pinned to member 3l, if desired. Nut 34 limits the travel of member 3|, or may be used by adding lock-washers to prevent member 3| from turning. Outlet connection 5 is held by packed gland 32-33 in movable hydraulic connection with member 3|. Thus, member 3| may be adjusted to any desired position to produce standing waves, or to produce any other interference pattern, the particular adjustment being selected to produce the most effective pattern of energy distribution in the :duid under treatment.

If reiiector crystal I5 is one-half the thickness of the vibratory crystals, its rear surface will be a nodal point and little or no mechanical motion will occur where it engages electrode 2|. This results in a minimization of energy developed at the rear terminus of the crystal stack with a corresponding increase in the amount of energy transmitted to the iluid under treatment. For example, let t1 represent the thickness of each of the vibratory crystals II, I2, and I3 and the effective thickness of front plate 6, and let t2 represent the thickness of reector plate I5. As previously described t2: 1A, t1. At the fundamental frequency of the intermediate crystals 7\ tl-- where A represents the wavelength of the mechanical vibrations generated in the crystals. Then where higher frequencies are desired, odd harmonies of the fundamental frequency may be employed and m and n being any positive integers.

Reflector crystal I5 has been indicated as a crystal of material having the same vibratory properties as the other crystals of the stack. It may be desirable in many instances to substitute some other substance, such as steel or other metallic alloy. By using a substance which is electrically conducting, the necessity for aluor otherwise rendering the reflector plate electrically conducting will be eliminated. The thickness of the rear reflector member, when constructed of 'some substance dissimilar in its properties from the vibratory crystals, will depend upon the velocity of propagation of the mechanical vibrations through the particular substance, the thickness being such that it will be substantially one-quarter wavelength, or an odd multiple oi quarter wavelengths, assuming I boatsr the vibratory crystals to be one-half wavelength in thickness or an odd number of half wavelengths in thickness respectively. If there is a change in propagation velocity accompanying a change in frequency, and it isdesired to use the same material at a. diierent frequency, the thickness may be adjusted accordingly. For best results, it is desirable that the radiation resistance (product of density and acoustic velocity) of the material used in the reflector be substantially the same as that of quartz. This maintains a uniformity of medium and avoids undesired reection phenomena. It is also desirable that mechanical hysteresis losses of the reector be of the same order of magnitude as that of the vibratory crystals in order to obtain effective reection.

The crystals have been indicated as circular discs, but any other convenient shapev may be utilized if desired.

The embodiment of the' invention described above is particularly adapted for the treatment of iluids by high frequency acoustic energy. The invention, however, by reason of the well known piezo-electric properties of the crystals, will operate eiectively t transform acoustic energy into electrical energy. When so operating, the reector member I serves to improve the eiciency of the transducer in the same manner as when the transducer is delivering an output of mechanical energy.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. In a piezo-electric transducer, a metallic housing comprising two sections, a plate of piezoelectric material yieldingly clamped between said sections, annular grooves on both sides of said plate circumscribing a central area thereof and increasing the freedom of movement of said central area with respect to the remainder of said plate, a plurality of disc-shaped piezo-electric members of substantially identical dimensions and piezo-electric properties stacked upon each other and in contact with one side of said circumscribed area of said plate, a terminating piezoelectric disc in contact with said stack ofdiscs of substantially the same diameter and piezoelectric properties but one half the thickness of said first-named discs, an electrically conducting coating completely surrounding said terminating disc, an electrode yieldingly engaging said conducting coating, a source of high-frequency energy having two terminals, conductors connecting one of said terminals to said electrode and the other of said terminals to said metallic housing, means for circulating an electric conductive cooling and insulating iiuid in and about the section of said housing containing said discs, and means for circulating a uid to be treated in and about the other section of said housing.

2. An electromechanical transducer comprising a housing, a principal piezo-electric crystal dividing the housing into two portions, means for circulating a uid through one portion, a plurality of intermediate piezo-electric crystals in the other portion and in contact with the principal crystal and an electrical circuit disposed to permit energization of all of the crystals simultaneously whereby mechanical vibrations may be imparted to the fluid directly from the principal crystal.

3. An electro-mechanical transducer including a plurality of piezo-electric crystals of identical material ln stacked relation and comprising an outer crystal of substantially one-half the thickness .of the other crystals and having a metallic coating disposed to render the outer crystal substantially piezo-electrically inactive.

4. An electro-mechanical transducer including a plurality of piezo-electric crystals in stacked relation and an outer harmonically related inactive crystal of the same material as the rstnamed crystals having a nodal outer surface.

5. An electro-mechanical transducer comprising a metallic housing, a piezo-electric plate dividing the housing into a fluid treating compartmentand a cooled transducing compartment, a plurality of disc-shaped piezo-electric crystals in stacked relationship, one of which is in contact with the plate, an electrically conductivev reilecting terminating disc of material having acoustic properties substantially equivalent to the properties of said piezo-electric crystals and having one side in contact with the crystal most remote from the plate and providing a nodal surface on the other side, and an electrical circuit including in series the terminating disc, the crystals, the plate, the uid to be treated and the housing and adapted to be energized from a suitable source of electrical energy.

6. An electro-mechanical transducer comprising a metallic housing, a piezo-electric plate dividing the housing into a treating compartment for an electrically conductive uid and a transducing and cooling compartment and in direct contact with the uid during treatment thereof, a plurality of disc-shaped piezo-electric' crystals in stacked relation one of them being in contact with the plate, a terminating disc of piezo-electric material covered with an electrically conducting coating rendering it piezo-electrically inactive and substantially one-half the thickness of each of the crystals and in contact with the crystal most remote from the plate and an electrical circuit adapted to be energized from a suitable source including in series the terminating disc, the crystals, the plate, the fluid to be treated and the housing.

7. An' electro-mechanical transducer comprising a metallic housing, a piezo-electric plate dividing the housing into a treating compartment for an electrically conductive uid anda transducing compartment, a plurality of discshaped piezo-electric crystals in stacked relationship one of which is in contact with the plate, a mechanically antiresonant electrically conductive reiiecting termination of such material and dimensions that the time for wave propagation through the termination is substantially onehalf the time forpropagation through one of the crystals and an electrical transducer energization circuit including in series the termination, the crystals, the plate, and the iiuid to be treated.

8. An electro-mechanical transducer comprising a metallic housing, a piezo-electric plate disposed to have one side thereof in contact with a iiuid to be treated and dividing the housing into a fluid treating compartment and another compartment, a plurality of disc-shaped piezoelectric crystals in the other compartment in stacked relation having an end crystal in contact with the plate, and a piezo-electrically inactive reflecting termination in contact with the end crystal at the opposite end of such material and dimensions that the time for wave propagation through the termination is substantially onehalf the time for propagation through one of the crystals.

9. An electro-mechanical transducer comprising a two-part housing of electrically conductive material, a piezo-electric plate effectively of substantially an odd number of half-wavelengths in thickness with reference to the propagation velocity of vibration therein dividing the parts of the housing into a fiuid treating compartment and a transducing compartment but substantially electrically and mechanically insulated from the walls of the housing, a plurality of disc-shaped vibratory crystals disposed in the transducing compartment in stacked relation, one crystal being in contact with the plate and being of an odd number of half wavelengths in thickness, a piezo-electrically inactive reflector member in contact with the one of said vibratory crystals most remote from the plate and being effectively of an odd number of one-quarter wavelengths in thickness, an electric circuit, and circuit means adapted for connection to a suitable source of electrical energy capable of energizing all of the active piezo-electric elements at a frequency at or in proximity to the fundamental natural period of vibration of said vibratory crystals or a harmonic thereof.

10. A transducer as in claim 6 in which the terminating disc is rendered piezo-electrically inactive by electrostatic shielding means.

11. A transducer as inclaim 6 in which the terminating disc is rendered piezo-electrically inactive by a covering of aluminum applied thereto.

12. A transducer as in claim 2 in which the fluid to be treated is electrically conductive and the fiuid is included in the electrical circuit.

13. In a device of the class described, a piezoelectric driving member rendered broadly tuned by the driven load, and a sharply mechanically anti-resonant terminating member of the same material as the first-named member in mechanical engagement with the driving member.

14. In a device of the class described, electromechanical transducing means having two spaced active surfaces, a driven mechanical load in operative engagement with one surface, and a mechanically anti-resonant reflecting member of the same material as the first-named member in operative engagement with the other face.

15. A device as in claim 14 in which the mechanical load absorbs energy sufficient to render the tuning of the transducing means relatively broad and in which the reflecting member absorbs relatively little energy rendering its tuning relatively sharp.

16. An electro-mechanical transducer including a plurality of piezo-electric crystals in stacked relationship, the effective thickness of each crystal being substantially an odd number of half wavelengths and a piezo-electrically inactive reflecting member of the same material as the piezo-electric crystals in engagement with one end of the stack.

17. In an electromechanical transducer, in combination, a piezoelectrically active transducing member, and a piezoelectrically inactive reflecting member of the same material as the firstnamed member mechanically resonant at a frequency bearing a predetermined relationship to the natural frequency of the transducing member and in mechanically effective engagement therewith.

18. A transducer as in claim 17 in which a fluid film is interposed between the two members.

19. A transducer asin claim 17 in which a film of oil is interposed between the two members.

20. A transducer as in claim 17 in which an electrically non-conductive uld surrounds the transducing member and forms a film extending intermediate the two members.

21. In an electromechanical transducer for operation at a predetermined frequency, in combination, a piezoelectrically active transducing member of the same material as the transducing member, and a piezoelectrically inactive reflecting member in effective mechanical engagement with the transducing member and of dimensions so related to the dimensions of the transducing member, that at the frequency of operation of the transducer the reflecting member will provide a surface at which mechanical vibrations imparted to the reflecting member by the transducing member are minimized.

22. A transducer as in claim 21 in which. a fluid film is interposed between the two members.

23. A transducer as in claim 21 in which a film of oil is interposed between the two members.

24. A transducer as in claim 21 in which an electrically non-conductive fiuld surrounds the transducing member and forms a fllm extending intermediate the two members.

25. A transducing device comprising two compartments, a piezoelectrically active memberhaving two spaced surfaces forming a partition between the two compartments, and a further piezoelectrically active member in one of the compartments having a surface in effective mechanical engagement with a surface of the partition member.

26. A device as in claim 25 in which a fluid film is interposed between the engaging surfaces of the two members.

27. A device as in claim 25 in which a film of oil is interposed between the engaging surfaces of the two members.

28. A device as in claim 25 in which an electrically non-'conductive fluid surrounds the further member and forms a film extending between the engaging surfaces of the two members.

29. A transducing device comprising two compartments, a piezoelectrically active member having two spaced surfaces forming a partition between the two compartments, a 'piezoelectrically active transducing member4 in one of the compartments having two spaced surfaces. one of which is in mechanically effective engagement with a surface of the partition member, and a piezoelectrically inactive reflector member having two spaced surfaces one of which is in mechanically effective engagement with a surface of a piezoelectrically active member the other surface of the reflector member being substantially vibrationally nodal and suitable for engagement with a fixed supporting member.

30. A device as in claim 29 in which a fluid film is interposed between the engaging surfaces of the members.

31. A device as in claim 29 in which a film of oil is interposed between the engaging surfaces of the members.

32. A device as in claim 29 in which an electrically non-conductive fluid surrounds the transducing member and forms a film extending over the engaging surfaces thereof.

33. In an electromechanical transducer, in combination, a piezoelectrically active transducing member having two spaced substantially parallel surfaces, anda piezoelectrically inactive reflector member of the same material as the being in eective mechanical engagement with one of the surfaces of the transducng member y and the other surface being substantially Vibrationally nodal and adapted for engagement with a xed surface.

34. A 'transducer as in claim 33 in 'which a uid lm is interposed between the two members.

35. A transducer as in claim 33 in which a lm of oil ,is interposed between the two members.

36. A transducer as in claim 33 in which an electrically non-conductive uid surrounds the transducing member and forms a lm extending intermediate the engaging surfaces of the two members.

` CORA E. HOLDEN,

Ezeeutri under the Last'Will and Testament of Charles Revell Holden, Deceased.

REFERENCES @man The following references are of record in the file vof this patent:

UNITED STATES PATENTS Number Name Date 1,734,926 Tripp Nov.4, 1929 1,959,429 Hovgard May 22, 1934 2,138,051 Williams Nov. 29, 1938 2,163,649 Weaver June 27, 1939 2,323,030 Greutzmacher June 29, 1943 2,327,487 Bach Aug. 24, 1943 ertificate of Correction Patent No. 2,498,737 February 28, 1

CHARLES REVELL HOLDEN, DECEASED It is hereby certified that errors appear in the printed specification of the ab numbered patent requiring correction as follows:

Column 5, lines 57 and 58, strike out the Words an electric conductive insert instead a; line 60, for a before fluid read am electrically conductive;

and that the said Letters Patent should be read with these corrections therein i l the' 'same-may conform to the record of the case in the Patent Ofce.

W'Spigned and sealed this 11th day of July, A. D. 1950.

[SEAL] THOMAS F. MURPHY,

Assistant 'ommzssz'oner of Patent Certificate of Correction Patent No. 2,498,737 February 28, 1950 CHARLES REVELL HOLDEN, DECEASED It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction es follows:

Column 5, lines 57 and 58, strike out the words an electric conductive and insert instead a; line 60, for a before fluid read an electrically conductive;

and that the seid Letters Patent should be reed with these corrections therein that thsamermay conform to the record of the case in the Patent Ofce.

WMSdgmed and sealed this 11th day of July, A. D. 1950.

[SEAL] THOMAS F. MURPHY,

Assistant Commissioner of Patents. 

